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Lichen it up in the trees

My favorite outdoor activity as a child was to climb high into the branches of a spruce tree, hack off a couple branches, and build myself a fort. But it wasn’t until the fall of 2012 that I realized I could climb trees as an adult, too. And, even better, I could call it ‘research’. As a PhD student, I studied how the environment influences epiphytic lichen communities across a variety of spatial scales. My first foray into the field involved tromping around the woods of North Carolina (USA) cataloging the morphological diversity of lichens living on the trunk of trees, but I soon became frustrated by the lack of appropriate subjects in the understory when a bounty of lichenological diversity appeared to mock me from the fallen twigs that I would pick up from the ground.

A. My trunk samples. (Can you see the giant spider?) B. Lichens on twigs from the canopy. (Not in my samples)

But then I thought, ‘why not just go up where the twigs are?’ Tree canopies are an excellent place to investigate how small-scale environmental differences affect multi-species communities. So, I took a class, bought a rope, grabbed some data loggers and headed up. Climbing trees in closed canopy forests is hard. For one, climbable trees large enough to support a bouncing 200lb primate, but with branches low enough that a line can be thrown over (either by hand or with the assistance of a giant slingshot) are not easy to come by. Choosing a large common tree species (like Northern Red Oak) was vital. I also learned very quickly to always carry duplicate equipment bedecked in hot pink flagging because 1) there’s nothing worse than unnecessary climbing in 32°C at 95% humidity and 2) nothing drops straight down (especially not an hour’s worth of lichen samples in small brown paper bags that flutter down to a forest floor covered in dried oak leaves).

The long drop.

Variability in temperature and humidity can change rapidly through 20 m of foliage and light levels can vary from one side of a branch to another. But, do these environmental differences actually limit which lichen species establish and persist? Not necessarily. If lichen species are able to adjust their physiology to cope with a wide variety of conditions, then the same species might be able to live throughout the canopy, but individuals would have different physical attributes depending on the stresses imposed in different locations. In order to detect whether environmental factors were affecting individuals I decided to measure several traits related to a lichen’s ability to capture light and retain water (the two main constraints on their growth). The basic idea was to test A) whether different species live in different environments in the canopy and if not, B) whether light and humidity constrain the physical attributes of individuals. Turns out that the answer is B, but not that much. No matter how I looked at the data (and I looked at it in a lot of different ways), species composition was barely affected by the environment and lichens living under different conditions had only slightly different physical attributes. Lichens are amazingly persistent and flexible composite organisms. I guess I shouldn’t be surprised that they can handle the North Carolina Piedmont canopy, no sweat. Other forests? Well, that may be a completely different story.

Jes Coyle

Punctelia rudecta was a very common macrolichen in all samples.

Linoleum and wire makes a flexible quadrat for sampling on branches.

Film-canister-sized HOBO data loggers record light and temperature at locations where I sampled lichens.

Thanks are due to Duke Forest and Waterdog Farms for tree access and to NSF for providing funds to quadruple my sample size and support an amazing field assistant. (Thanks to my industrious trait-measurers as well.)

*A version of this essay originally appeared on Biodiverse Perspectives on July 9, 2013.

Oikos Journal

Oikos is a journal issued by the Nordic Ecological Society and is one of the leading peer-reviewed journals in ecology. Oikos publishes original and innovative research on all aspects of ecology, defined as organism-environment interactions.